Wiring substrate

ABSTRACT

An object of the invention is to prevent the color on a surface of a plated metal layer from changing. The invention is a wiring substrate obtained by forming a wiring conductor made of a metal having a high melting point on an insulator, and coating a surface of the wiring conductor with an electroless plated metal layer, wherein the electroless plated metal layer contains an element of Group 1B and is free from lead.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wiring substrate on which anelectronic component such as a semiconductor element, a capacitiveelement, a resistor or the like is mounted, and that includes a wiringconductor on its surface, the wiring conductor being coated with aplated metal layer by electroless plating. The invention also relates toa method for producing the same.

2. Description of the Related Art

Conventionally, a wiring substrate on which an electronic component suchas a semiconductor element, a capacitive element, a resistor or the likeis to be mounted generally includes a substantially rectangularplate-shaped insulator and a plurality of wiring conductors. Theinsulator is made of an aluminum oxide sintered body and has a mountingportion for mounting an electronic component. The plurality of wiringconductors are made of a metal material having a high melting point suchas tungsten, molybdenum or manganese are formed so as to lead from themounting portion to the outer circumference of the insulator. Anelectronic component such as a semiconductor element, a capacitiveelement and a resister is mounted on the mounting portion of theinsulator, and each electrode of the electronic component iselectrically connected to the wiring conductor via a conductiveconnecting member such as a solder and a bonding wire.

In such a wiring substrate, a portion of the wiring conductor that isled out to the outside is connected to a circuit wiring of an externalelectrical circuit board via a solder or the like, so that the wiringsubstrate is mounted on the external electrical circuit, and eachelectrode of the electronic component mounted on the wiring substrate iselectrically connected to a predetermined external electrical circuit.

Furthermore, in such a wiring substrate, the surface of the wiringconductor is coated with a plated metal layer such as nickel to improvethe wettability and the bonding properties of solders and bonding wireswith respect to the wiring conductor made of a metal material having ahigh melting point.

As a method for forming a coating of a plated metal layer such asnickel, electroless plating, which does not require leads, has come tobe used more commonly, because higher density of the wiring conductorsresulting from compactness of the wiring substrate makes it difficult toform leads for supplying power for plating.

A coating of a plated metal layer such as nickel on the wiringconductors by electroless plating is formed by the following method.Since metals having a high melting point such as tungsten, molybdenum ormanganese do not have catalytic activity with respect to reductionprecipitation of metals such as nickel by electroless plating(autocatalytic type), in general, first, the surface of the wiringconductor is coated with an element having catalytic activity withrespect to precipitation of nickel or the like, such as an element ofGroup 1B (copper, silver, or gold), to provide the surface thereof withcatalytic activity. Thereafter, the wiring conductor is immersed in anelectroless plating solution so that the surface is coated with a platedmetal layer by the action of the catalytic activity of the element ofGroup 1B. In general, this method is performed in the following manner.

First, an insulator having wiring conductors on its surface is prepared.

Next, the wiring conductors are immersed in an active liquid obtained byadding additives such as a pH adjuster and a complexing agent to anaqueous solution containing at least a metal compound that serves as asupply source of an element of Group 1B such as copper chloride, and alead compound such as lead chloride, so that the element of Group 1Bsuch as copper is precipitated on the surface of the wiring conductorfor coating. Then, the wiring conductor is immersed in an active liquidobtained by adding additives such as a complexing agent and a pHadjuster to an aqueous solution containing at least a metal compoundthat serves as a supply source of a plated metal such as nickel sulfideor nickel chloride, and a lead compound such as lead chloride, so thatthe element of Group 1B such as copper is precipitated on the surface ofthe wiring conductor for coating.

Then, the wiring conductors are immersed in an electroless platingsolution obtained by adding a completing agent, a pH buffer, astabilizer and the like to an aqueous solution containing at least anickel compound that serves as a supply source of nickel such as nickelsulfide or nickel chloride, and a reductant such as sodium hypophosphiteor dimethylamine borane, so that the nickel in the plating solution isreduced and precipitated by the action of the catalytic activity of theelement of Group 1B such as copper that is coating the surfaces of thewiring conductors, so that a nickel plated metal layer is formedselectively only on the surfaces of the wiring conductors.

The lead compound contained in the active liquid is first adsorbed onthe surface of the wiring conductor when the wiring conductors made of ametal having a high melting point are immersed in the active liquid, andthe lead compound acts so as to make the surfaces of the conductorssensitive to the precipitation and the coating of the element of Group1B such as copper. Thus, the lead compound facilitates the precipitationand the coating of the element of Group 1B such as copper on the wiringconductors and makes it possible to form a uniform coating. Furthermore,the element of Group 1B such as copper coating the surface of the wiringconductor and the lead remain and are contained in the electrolessplated metal layer coating the surfaces of the wiring conductors.

Another method for forming a coating of a plated metal layer such asnickel and copper on the wiring conductor by electroless plating is asfollows. Since metals having a high melting point such as tungsten,molybdenum or manganese do not have catalytic activity with respect toreduction and precipitation by electroless plating (autocatalytic type)of metals such as nickel or copper, in general, first, the surface ofthe wiring conductor is coated with a platinum group metal such aspalladium or platinum to provide the surface thereof with catalyticactivity. Thereafter, the wiring conductor is immersed in an electrolessplating solution so that the surface is coated with a plated metallayer. In general, this method is performed in the following manner.

First, an insulator having wiring conductors on its surface is prepared.

Next, the wiring conductors are immersed in an active liquid obtained byadding additives such as a pH adjuster such as sodium hydroxide orpotassium hydroxide to an aqueous solution containing at least a metalcompound that serves as a supply source of a platinum group metal suchas palladium chloride, and a lead compound such as lead chloride, sothat the platinum group metal such as palladium is precipitated on thesurface of the wiring conductor for coating.

Then, the wiring conductors are immersed in an active liquid obtained byadding a complexing agent, a pH buffer, a stabilizer and the like to anaqueous solution containing at least a metal compound that serves as asupply source of a plated metal such as nickel sulfide or coppersulfide, and a reductant such as sodium hypophosphite, dimethylamineborane, formalin, so that a metal such as nickel or copper is reducedand precipitated by the action of the catalytic activity of the platinumgroup metal such as palladium coating the surfaces of the wiringconductors, so that a plated metal layer is formed selectively only onthe surfaces of the wiring conductors.

The lead compound contained in the active liquid is first adsorbed onthe surface of the wiring conductor when the wiring conductors made of ametal having a high melting point is immersed in the active liquid, andthus the lead compound acts so as to make the surfaces of the conductorssensitive to the precipitation and the coating of the platinum groupmetal such as palladium. Thus, the lead compound facilitates theprecipitation and the coating of the platinum group metal such aspalladium or platinum on the wiring conductors and makes it possible toform a uniform coating. Furthermore, the platinum group metal such asplatinum or palladium coating the surface of the wiring conductor andthe lead remain and are contained in the electroless plated metal layercoating the surfaces of the wiring conductors.

The conventional wiring substrate contains lead in the plated metallayer, as described above, so that when heat is applied to the platedmetal layer, the lead moves and is diffused onto the surface of theplated metal layer and oxidized there, which causes a functional problemthat the color is changed spottedly and environmental and securityproblems that the lead in the plated metal layer harms the human body.

Furthermore, it can be one solution not to let lead contained in theactive liquid in order to solve the problems. However, in this case,since the surface of the wiring conductor made of a metal having a highmelting point is not sufficiently sensitive to the precipitation and thecoating of elements of Group 1B such as copper or platinum group metalssuch as palladium or platinum, the elements of Group 1B cannot beprecipitated on the surfaces of the wiring conductors uniformly andfirmly for coating. As a result, roughness, cracks, swelling or the likemay be generated.

SUMMARY OF THE INVENTION

The invention is devised to solve the problems, and an object of theinvention is to provide a wiring substrate in which a wiring conductoris coated with an electroless plated metal layer uniformly and firmly,and the plated metal layer is free from lead so that the problem of aspotted color change is not caused, and the human body is not harmed.

The invention relates to a wiring substrate obtained by forming a wiringconductor made of a metal having a high melting point on an insulator,and coating a surface of the wiring conductor with an electroless platedmetal layer, wherein the electroless plated metal layer contains anelement of Group 1B and is free from lead.

The invention also relates to a method for producing a wiring substrate,comprising the steps of:

-   -   (1) preparing an insulator in which a wiring conductor made of a        metal having a high melting point is formed on its surface;    -   (2) immersing the wiring conductor in an active liquid        containing at least an element of Group 1B and a hydroxy        carboxylic acid to coat a surface of the wiring conductor with        the element of Group 1B to provide catalytic activity; and    -   (3) immersing the wiring conductor in an electroless plating        solution to coat the surface of the wiring conductor with an        electroless plated metal layer.

According to the invention, the electroless plated metal layer coatingthe wiring conductor contains an element of Group 1B that is necessaryto form a coating of the electroless plated metal layer, but does notcontain lead. Therefore, the function of the element of Group 1Bprovides the wiring conductor with satisfactory catalytic activity, sothat only the wiring conductor is coated with the electroless platedmetal layer uniformly, and the problems of a spotted color change of theplated metal layer caused by the lead contained in the plated metallayer and the harm to the human body can be prevented effectively.

In the invention it is preferable that the insulator is made of analuminum oxide sintered body, an aluminum nitride sintered body, amullite sintered body, or silicon carbide sintered body, the wiringconductor is made of tungsten, molybdenum, or manganese, the electrolessplated metal layer is made of a high purity nickel containing nickel ina content of 99.9 wt % or more, a nickel-phosphorus alloy or anickel-boron alloy.

In the invention, it is preferable that the surface of the electrolessplated metal layer is coated with a gold plated layer.

In the invention, it is preferable that the insulator is made of analuminum oxide sintered body, an aluminum nitride sintered body, amullite sintered body, or silicon carbide sintered body, and the wiringconductor is made of tungsten, molybdenum, or manganese.

In the invention, it is preferable that the element of Group 1B is atleast one selected from the group consisting of copper, silver and gold,the hydroxy carboxylic acid is at least one selected from the groupconsisting of citric acid and malic acid, the electroless plated metallayer is made of a high purity nickel containing nickel in a content of99.9 wt % or more, a nickel-phosphorus alloy or a nickel-boron alloy.

In the invention, it is preferable that the concentration of the elementof Group 1B in the active liquid is 20 to 80 ppm.

The invention relates to a wiring substrate obtained by forming a wiringconductor made of a metal having a high melting point on an insulator,and coating a surface of the wiring conductor with an electroless platedmetal layer, wherein the electroless plated metal layer contains aplatinum group metal and is free from lead.

The invention also relates to a method for producing a wiring substrate,comprising the steps of:

-   -   (1) preparing an insulator including a wiring conductor made of        a metal having a high melting point on its surface;    -   (2) immersing the wiring conductor in an active liquid        containing at least a platinum group metal and a hydroxy        carboxylic acid to coat a surface of the wiring conductor with        the platinum group metal to provide catalytic activity; and    -   (3) immersing the wiring conductor in an electroless plating        solution to coat the surface of the wiring conductor with an        electroless plated metal layer.

According to the invention, the electroless plated metal layer coatingthe wiring conductor contains a platinum group metal that is necessaryto form a coating of the electroless plated metal layer, but does notcontain lead. Therefore, the function of the platinum group metalprovides the wiring conductor with satisfactory catalytic activity, sothat only the wiring conductor is coated with the electroless platedmetal layer uniformly, and the problems of a spotted color change of theplated metal layer caused by the lead contained in the plated metallayer and the harm to the human body can be prevented effectively.

In the invention, it is preferable that the insulator is made of analuminum oxide sintered body, an aluminum nitride sintered body, amullite sintered body, or silicon carbide sintered body, the wiringconductor is made of tungsten, molybdenum, or manganese, the electrolessplated metal layer is made of a high purity nickel containing nickel ina content of 99.9 wt % or more, a nickel-phosphorus alloy, anickel-boron alloy, copper, or an alloy containing copper as a maincomponent.

In the invention, it is preferable that the surface of the electrolessplated metal layer is coated with a gold plated layer.

In the invention, it is preferable that the insulator is made of analuminum oxide sintered body, an aluminum nitride sintered body, amullite sintered body, or silicon carbide sintered body, and the wiringconductor is made of tungsten, molybdenum, or manganese.

In the invention, it is preferable that the platinum group metal is atleast one selected from the group consisting of palladium, platinum,rhodium, ruthenium, and iridium, the hydroxy carboxylic acid is at leastone selected from the group consisting of citric acid and malic acid,the electroless plated metal layer is made of a high purity nickelcontaining nickel in a content of 99.9 wt % or more, a nickel-phosphorusalloy or a nickel-boron alloy.

In the invention, it is preferable that the concentration of theplatinum group metal in the active liquid is 20 to 80 ppm.

BRIEF DESCRIPTION OF THE DRAWINGS

Other and further objects, features, and advantages of the inventionwill be more explicit from the following detailed description taken withreference to the drawings wherein:

FIG. 1 is a sectional view showing an example of a wiring substrate of afirst embodiment of the invention;

FIG. 2A is an enlarged sectional view showing an enlarged part of thewiring substrate shown in FIG. 1, and FIG. 2B is an enlarged sectionalview showing a part of a variation of the wiring substrate of the firstembodiment of the invention;

FIGS. 3A to 3C are enlarged sectional views of each process explaining amethod for producing the wiring substrate shown in FIG. 1;

FIG. 4A is an enlarged sectional view showing an enlarged part of awiring substrate of a second embodiment of the invention, and FIG. 4B isan enlarged sectional view showing a part of a variation of the wiringsubstrate of the second embodiment of the invention; and

FIGS. 5A to 5C are enlarged sectional views of each process explaining amethod for producing the wiring substrate of the second embodiment ofthe invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now referring to the drawings, preferred embodiments of the inventionare described below.

FIG. 1 is a sectional view showing an example of a wiring substrate of afirst embodiment of the invention that is applied to a semiconductorelement accommodation package for accommodating a semiconductor element.Reference numeral 1 denotes an insulator, and reference numeral 2 is awiring conductor. The insulator 1 and the wiring conductor 2 constitutea wiring substrate 4 for mounting a semiconductor element 3.

The insulator 1 is made of an electrically insulating material such asan aluminum oxide sintered body, an aluminum nitride sintered body, amullite sintered body, or silicon carbide sintered body, and has amounting portion for mounting the semiconductor element 3 on its uppersurface. The wiring conductor 2 made of a metal having a high meltingpoint such as tungsten, molybdenum, manganese and other various metalsis formed on the insulator 1 from the mounting portion on which thesemiconductor element 3 is to be mounted to the lower surface thereof.

In the insulator 1, the semiconductor element 3 is mounted on themounting portion and each electrode of the semiconductor element 3 iselectrically connected to the wiring conductor 2 that is exposed to themounting portion via a solder ball 5. The portions of the wiringconductor 2 led out onto the lower surface of the insulator 1 areelectrically connected to a circuit wiring of an external electricalcircuit substrate via solders or the like.

FIG. 2A is an enlarged sectional view showing an enlarged part of thewiring substrate 4 shown in FIG. 1. The surface of the wiring conductor2 is coated with a plated metal layer 6 formed by electroless plating,as shown in the sectional view of FIG. 2A.

The plated metal layer 6 serves to improve the wettability, the joiningstrength and the bonding properties of solders with respect to thewiring conductor 2 and is made of high purity nickel that containsnickel in a content of 99.9 wt % or more, a nickel-phosphorus alloy, anickel-boron alloy or the like.

It is important in the invention that the plated metal layer 6 containsan element of Group 1B and is free from lead.

This is important in order to prevent the problems that the leadcontained in the plated metal layer 6 such as a nickel-boron platedlayer moves and is diffused onto the surface of the plated metal layer 6by heat or the like so that the color is changed spottedly, or the leadharms the human body. In this case, the element of Group 1B (copper,silver, or gold) allows the wiring conductor 2 to be coated with theplated metal layer 6 by electroless plating, and the element remains andis contained in the plated metal layer 6. However, compared to lead, theelements of Group 1B are hardly oxidized, and the toxicity is very low,so that the elements of Group 1B cannot move and is diffused onto thesurface of the plated metal layer 6 so as to cause the spotted colorchange, nor harm the human body.

It is not necessary that the element of Group 1B is formed along thesurface of the wiring conductor 2 in the form of a film, and it issufficient that the element of Group 1B is formed uniformly at asubstantially constant interval in the form of grains or fragments, forexample, in portions along the grain boundaries the crystal grains ofthe metal having a high melting point of which the wiring conductor 2 ismade. The element of Group 1B serves as the initiating point to coat thesurface of the wiring conductor 2 with the plated metal layer 6uniformly.

FIG. 2B is an enlarged sectional view showing a part of a variation ofthe wiring substrate 4 of the invention. In the wiring substrate 4, whenthe surface of the plated metal layer 6 made of pure nickel thatcontains nickel in a content of 99.9 wt % or more, a nickel-phosphorusalloy, a nickel-boron alloy or the like is coated with a gold platedlayer 9, as shown in FIG. 2B, the plated metal layer 6 can be preventedfrom being oxidized and corroded effectively, and the wettability of thesolder with respect to the wiring conductor 2 can be improved further.Therefore, it is preferable that the in the wiring substrate 4, thesurface of the plated metal layer 6 is further coated with the goldplated layer 9. In this case, when the thickness of the gold layer 9 isless than 0.03 μm, the effect of coating the plated metal layer 6 isweak, and when the thickness exceeds 0.8 μm, a large amount of fragileintermetallic compounds is produced between tin in the solder and goldso that the joining strength of the solder tends to deteriorate.Consequently, it is preferable that the thickness of the gold platedlayer 9 is in the range from 0.03 μm to 0.8 μm.

In this manner, in the wiring substrate 4 of the invention, thesemiconductor element 3 is mounted on the mounting portion of theinsulator 1, and each electrode of the semiconductor element 3 iselectrically connected to the wiring conductor 2 via the solder ball 5.Thereafter, a bowl-shaped cover 7 made of a metal or ceramics is joinedonto the upper surface of the insulator 1 with a sealing material suchas glass, resin, wax material or the like so that the semiconductorelement 3 can be accommodated airtightly in a container constituted withthe insulator 1 and the cover 7. Thus, a semiconductor apparatus as aproduct can be completed.

Next, a method for producing the wiring substrate 4 will be describedwith reference to FIGS. 3A to 3C. The same components as those in FIGS.1 and 2A bear the same numeral.

First, an insulator 1 provided with a wiring conductor 2 made of a metalhaving a high melting point on its surface as shown in FIG. 3A isprepared.

The insulator 1 is a substantially rectangular plate made of anelectrically insulating material such as an aluminum oxide sinteredbody, an aluminum nitride sintered body, a mullite sintered body, asilicon carbide sintered body or the like, and has a mounting portionfor mounting a semiconductor element on its upper surface, so that thesemiconductor element is mounted on the mounting portion.

The insulator 1 is produced in the following manner in the case wherethe insulator 1 is made of an aluminum oxide sintered body. A suitablebinder and a solvent are mixed with a raw material powder such asaluminum oxide, silicon oxide, calcium oxide and magnesium oxide so thata muddy liquid ceramic slurry is obtained. Next, the ceramic slurry isformed into a sheet form by a conventionally known sheet formingtechnique such as a doctor blade technique, calender rolling or thelike, so that a ceramic green sheet (ceramic crude sheet) is obtained.Thereafter, the ceramic green sheet is formed into a suitable form bycutting or stamping processing, and a plurality of sheets are laminated.Finally, the laminated ceramic green sheet is fired at a temperature ofabout 1600° C. in a reducing atmosphere. Thus, the insulator 1 can beproduced.

The wiring conductor 2 is made of a metal having a high melting pointsuch as tungsten, molybdenum, and manganese. The wiring conductor 2 canbe formed from the mounting portion of the insulator 1 to the lowersurface thereof in the following manner. A metal paste is obtained byadding and mixing a suitable organic binder and a solvent to a metalpowder having a high melting point such as tungsten, and the metal pasteis previously applied to the ceramic green sheet that will become theinsulator 1 so that a predetermined pattern is printed by conventionallyknown screen printing.

Next, the wiring conductor 2 is immersed into an active liquidcontaining at least one of the elements of Group 1B consisting ofcopper, silver and gold and at least one hydroxy carboxylic acid such ascitric acid and malic acid, and as shown in FIG. 3B, the element 8 ofGroup 1B is formed on the surface of the wiring conductor 2 to providecatalytic activity. In FIG. 3B, the element of Group 1B is shownexaggerated than the actual scale for illustration.

In the active liquid, the element of Group 1B serves to provide thesurface of the wiring conductor 2 with catalytic activity by coating thesurface of the wiring conductor 2, and makes it possible to coatselectively and uniformly the surface of the wiring conductor 2 with theplated metal layer 6 in a subsequent process.

The hydroxy carboxylic acid has an important function of making itpossible to coat the surface of the wiring conductor 2 with the element8 of Group 1B without letting lead contained in the active liquid. Morespecifically, the hydroxy carboxylic acid such as citric acid acts onthe surface of the wiring conductor 2 made of a metal having a highmelting point such as tungsten so that the metal having a high meltingpoint in the surface portion of the wiring conductor 2 is oxidized andcomplexed so as to be eluted in the active liquid, and that the elementof Group 1B is reduced and precipitated by being substituted fortungsten or the like in the site from which the metal having a highmelting point has been eluted. It seems that this occurs for thefollowing reason: Regarding the stability of the complex with respect toa metal of the hydroxy carboxylic acid such as citric acid, thestability with respect to the metal having a high melting point such astungsten is larger than that with respect to the element of Group 1B.Thus, the hydroxy carboxylic acid is added to the active liquid, so thatthe surface of the wiring conductor 2 can be coated with the element ofGroup 1B easily and uniformly without adding lead as a sensitizing agentto the active liquid.

When copper, for example, is used as the element of Group 1B, an aqueoussolution containing at least a copper compound such as copper chlorideor copper sulfide, and a hydroxy carboxylic acid (carboxylic acid havinga hydroxyl group) such as citric acid or malic acid, to which additivessuch as a pH adjuster, for example, ammonia salts, hydrochloric acid,fluoroboric acid, sodium hydroxide, potassium hydroxide or the like areadded, can be used as the active liquid. It is preferable that theconcentration of the element of Group 1B in the active liquid is about20 to 80 ppm, because a high concentration may induce a problem such assegregation of the element of Group 1B.

Then, the wiring conductor 2 is immersed in an electroless platingsolution, so that the electroless plated metal layer 6 is precipitatedon the surface of the wiring conductor 2 for coating, using the element8 of Group 1B as a catalyst, as shown in FIG. 3C.

The plated metal layer 6 is made of pure nickel that contains nickel ina content of 99.9 wt % or more, a nickel-phosphorus alloy, anickel-boron alloy or the like, and serves to improve the wettability,the bonding properties or the like of the solder with respect to thewiring conductor 2.

When the electroless plated metal layer 6 is made of, for example, anickel-boron alloy, an electroless nickel plating solution containing atleast a nickel compound that serves as a nickel supply source such asnickel sulfide, and a boron-based reductant such as dimethylamine boron,to which a complexing agent, a stabilizer, a pH buffer or the like areadded, can be used as the electroless plating solution. In this case,the nickel ions in the electroless nickel plating solution are reducedto metal nickel, which occurs together with oxidation and degradation ofthe reductant by the catalytic action of the element 8 of Group 1B thathas been previously formed on the surface of the wiring conductor 2 forcoating. Then, the surface of the wiring conductor 2 is eutecticallycoated with the metal nickel together with boron produced from thedegradation of the reductant, so that the plated metal layer 6 made of anickel-boron alloy can be formed. Once that the surface of the wiringconductor 2 is started to be coated with nickel or the nickel-boronalloy, the coated nickel itself has catalytic activity for subsequentreduction and precipitation of nickel with the reductant. Therefore,even if the element of Group 1B, which is a catalyst, is not exposed inthe plating solution, or is not in contact, the reaction for reduction,precipitation and coating of nickel can be performed continuously.

As shown in FIG. 2B, when the surface of the plated metal layer 6 iscoated with the gold plated layer 9, the wiring conductor 2 coated withthe plated metal layer 6 can be immersed in a substitution typeelectroless gold plating solution containing at least a gold compoundsuch as gold cyanide potassium, and a complexing agent such asethylenediamine tetraacetic acid (EDTA) for a predetermined time.

Next, a wiring substrate of a second embodiment of the invention will bedescribed.

In this embodiment, the components corresponding to those in thestructure of the first embodiment bear the same reference numeral, andthe description thereof will be omitted.

In this embodiment, the structure of the wiring substrate is similar tothat of the first embodiment, and the aspect to be noted is that theelectroless plated metal layer contains a platinum group metal and isfree from lead.

FIG. 4A is an enlarged sectional view showing an enlarged part of thewiring substrate of the second embodiment of the invention. The surfaceof the wiring conductor 2 is coated with the plated metal layer 16 byelectroless plating, as shown in the sectional view of FIG. 4A. Theplated metal layer 16 serves to improve the wettability, the joiningstrength, and the bonding properties of solders with respect to thewiring conductor 2 and is made of a high purity nickel that containsnickel in a content of 99.9 wt %, a nickel-phosphorus alloy, anickel-boron alloy, copper, an alloy containing copper as a maincomponent or the like.

In the invention, it is important that the plated metal layer 16contains a platinum group metal and is free from lead.

This is important in order to prevent the problems that the leadcontained in the plated metal layer 16 such as a nickel-boron platedlayer moves and is diffused onto the surface of the plated metal layer16 by heat or the like so that the color is changed spottedly, or thelead harms the human body. In this case, the platinum group metal suchas palladium or platinum has a function of providing catalytic activitynecessary for the surface of the wiring conductor 2 to be coated withthe plated metal layer 16 by electroless plating, and the platinum groupmetal is precipitated on the surface of the wiring conductor 2 forcoating. At the same time, the platinum group metal remains and iscontained in the plated metal layer 16. However, compared with lead, theplatinum group metals are hardly oxidized, and the toxicity is very low,so that the platinum group metals cannot move and is diffused onto thesurface of the plated metal layer 16 so as to cause the spotted colorchange, nor harm the human body.

It is not necessary that the platinum group metal is formed along thesurface of the wiring conductor 2 in the form of a film, and it issufficient that the platinum group metal is formed uniformly at asubstantially constant interval in the form of grains or fragments, forexample, in portions along the grain boundaries the crystal grains ofthe metal having a high melting point of which the wiring conductor 2 ismade. The platinum group metal serves as the initiating point to coatthe surface of the wiring conductor 2 with the plated metal layer 16uniformly.

As the platinum group metal, palladium or platinum, in particular,palladium is preferable, because palladium or platinum can coat thesurface of the wiring conductor 2 made of a metal having a high meltingpoint satisfactorily, and can provide a satisfactory catalytic activityto form a coating of a metal such as nickel or copper by electrolessplating.

FIG. 4B is an enlarged sectional view showing a part of a variation ofthe wiring substrate of the second embodiment of the invention. In thewiring substrate, the surface of the plated metal layer 16 made of purenickel that contains nickel in a content of 99.9 wt % or more, anickel-phosphorus alloy, a nickel-boron alloy, a copper, an alloycontaining copper as a main component or the like is coated with a goldplated layer 19, the plated metal layer 16 can be prevented from beingoxidized and corroded effectively, and the wettability of the solderwith respect to the wiring conductor 2 can be improved further.Therefore, it is preferable that the in the wiring substrate 4, thesurface of the plated metal layer 16 is further coated with the goldplated layer 19. In this case, when the thickness of the gold layer 19is less than 0.03 μm. the effect of coating the plated metal layer 16 isweak, and when the thickness exceeds 0.8 μm, a large amount of fragileintermetallic compounds is generated between tin in the solder and goldso that the joining strength of the solder tends to deteriorate.Consequently, it is preferable that the thickness of the gold platedlayer 19 is in the range from 0.03 μm to 0.8 μm.

In this manner, in the wiring substrate of the invention, as shown inFIG. 1, the semiconductor element 3 is mounted on the mounting portionof the insulator 1, and each electrode of the semiconductor element 3 iselectrically connected to the wiring conductor 2 via the solder ball 5.Thereafter, a bowl-shaped cover 7 made of a metal or ceramics is joinedonto the upper surface of the insulator 1 with a sealing material suchas glass, resin, wax material or the like so that the semiconductorelement 3 can be accommodated airtightly in a container constituted withthe insulator 1 and the cover 7. Thus, a semiconductor apparatus as aproduct can be completed.

Next, a method for producing the wiring substrate will be described withreference to FIGS. 5A to 5C. The same components as those in FIGS. 1 and4A bear the same numeral.

First, an insulator 1 provided with a wiring conductor 2 made of a metalhaving a high melting point on its surface as shown in FIG. 5A isprepared.

The insulator 1 is a substantially rectangular plate made of anelectrically insulating material such as an aluminum oxide sinteredbody, an aluminum nitride sintered body, a mullite sintered body, asilicon carbide sintered body or the like, and has a mounting portionfor mounting a semiconductor element on its upper surface, so that thesemiconductor element is mounted on the mounting portion.

The insulator 1 can be produced in the same manner as in the firstembodiment, so that the description thereof will be omitted.

The wiring conductor 2 can be formed in the same manner as in the firstembodiment, so that the description thereof will be omitted.

Next, the wiring conductor 2 is immersed into an active liquidcontaining at least one of the platinum group metals consisting ofpalladium, platinum, rhodium, ruthenium, and iridium, and at least onehydroxy carboxylic acid such as citric acid and malic acid, and as shownin FIG. 5B, the platinum group metal 18 is formed on the surface of thewiring conductor to provide catalytic activity. In FIG. 5B, the platinumgroup metal 18 is shown exaggerated than the actual scale forillustration.

In the active liquid, the platinum group metal serves to provide thesurface of the wiring conductor 2 with catalytic activity by coating thesurface of the wiring conductor 2 with the platinum group metal, andmakes it possible to coat selectively and uniformly the surface of thewiring conductor 2 with the plated metal layer 16 in a subsequentprocess.

The hydroxy carboxylic acid has an important function of making itpossible to coat the surface of the wiring conductor 2 with the platinumgroup metal 18 without letting lead contained in the active liquid. Morespecifically, the hydroxy carboxylic acid such as citric acid acts onthe surface of the wiring conductor 2 made of a metal having a highmelting point such as tungsten so that the metal having a high meltingpoint in the surface portion of the wiring conductor 2 is oxidized andcomplexed so as to be eluted in the active liquid, and that the platinumgroup metal is reduced and precipitated by being substituted fortungsten or the like in the site from which the metal having a highmelting point has been eluted. It seems that this occurs for thefollowing reason: Regarding the stability of a complex of the organicacid such as citric acid with respect to a metal, the stability withrespect to the metal having a high melting point such as tungsten islarger than that with respect to an activating agent such as theplatinum group metal. Thus, the hydroxy carboxylic acid is added to theactive liquid, so that the surface of the wiring conductor 2 can becoated with the platinum group metal easily and uniformly without addinglead as a sensitizing agent to the active liquid.

When palladium, for example, is used as the platinum group metal, anaqueous solution containing at least a palladium compound such aspalladium chloride or palladium sulfide, and a hydroxy carboxylic acid(carboxylic acid having a hydroxyl group) such as citric acid or malicacid, to which additives such as a pH adjuster, for example,hydrochloric acid, fluoroboric acid, sodium hydroxide, potassiumhydroxide or the like are added, can be used as the active liquid. It ispreferable that the concentration of the palladium in the active liquidis about 20 to 80 ppm, because a high concentration may induce a problemsuch as segregation of the palladium.

Furthermore, in order to achieve the substitution of tungsten withpalladium by the action of the hydroxy carboxylic acid effectively, itis preferable that the active liquid is acidic, particularly preferablyin the range of pH 1 to 3. In order to adjust the pH of the activeliquid to be in the predetermined range, pH adjusters such ashydrochloric acid, fluoroboric acid, sodium hydroxide, and potassiumhydroxide can be added to the active liquid, as appropriate.

Then, the wiring conductor 2 is immersed in an electroless platingsolution, so that the electroless plated metal layer 16 is precipitatedon the surface of the wiring conductor 2 for coating, using the platinumgroup metal as a catalyst.

The plated metal layer 16 is made of pure nickel that contains nickel ina content of 99.9 wt % or more, a nickel-phosphorus alloy, anickel-boron alloy, copper, an alloy containing copper as a maincomponent or the like, and serves to improve the wettability, thebonding properties or the like of the solder with respect to the wiringconductor 2.

When the electroless plated metal layer 16 is made of, for example, anickel-boron alloy, an electroless nickel plating solution containing atleast a nickel compound that serves as a nickel supply source such asnickel sulfide, and a boron-based reductant such as dimethylamine boron,to which a complexing agent, a stabilizer, a pH buffer or the like areadded, can be used as the electroless plating solution. In this case,the nickel ions in the electroless nickel plating solution are reducedto metal nickel, which occurs together with oxidation and degradation ofthe reductant by the catalytic action of the platinum group metal thathas been previously formed on the surface of the wiring conductor 2 forcoating. Then, the surface of the wiring conductor 2 is eutecticallycoated with the metal nickel together with boron produced from thedegradation of the reductant, so that the plated metal layer 16 made ofa nickel-boron alloy can be formed. Once that the surface of the wiringconductor 2 is started to be coated with nickel or the nickel-boronalloy, the coated nickel itself has catalytic activity for subsequentreduction and precipitation of nickel by the reductant. Therefore, evenif the platinum group metal, which is a catalyst, is not exposed in theplating solution, or is not in contact, the reaction for reduction,precipitation and coating of nickel can be performed continuously.

As shown in FIG. 4B, when the surface of the plated metal layer 16 iscoated with the gold plated layer 19, the wiring conductor 2 coated withthe plated metal layer 16 can be immersed in a substitution typeelectroless gold plating solution containing at least a gold compoundsuch as gold cyanide potassium, and a complexing agent such asethylenediamine tetraacetic acid (EDTA) for a predetermined time.

The wiring substrate of the invention is not limited to the embodimentsdescribed above, and it is possible to make various changes as long asthe changes do not depart from the gist of the invention. For example,in the embodiments, the wiring substrate of the invention is applied toa semiconductor element accommodation package for accommodating asemiconductor element, but the wiring substrate of the invention can beapplied to other applications such as a hybrid integrated circuit.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription and all changes which come within the meaning and the rangeof equivalency of the claims are therefore intended to be embracedtherein.

1. A wiring substrate comprising: an insulator; a wiring conductor madeof a metal having a high melting point formed on the insulator; anelement of Group 1B precipitated on a surface of the wiring conductor;and a lead-free electroless plated metal layer formed on the wiringconductor covering the element of Group 1B existing on, the surface ofthe wiring conductor, wherein the element of Group 1B has served as aninitiating point to coat the surface of the wiring conductor with thelead-free electroless plated metal layer, the element of Group 1B isprecipitated in a site from which the metal having a high melting pointon the surface of the wiring conductor has been eluted.
 2. The wiringsubstrate of claim 1, wherein the insulator is made of an aluminum oxidesintered body, an aluminum nitride sintered body, a muillite sinteredbody, or silicon carbide sintered body, the wiring conductor is made oftungsten, molybdenum, or manganese, the lead-free electroless platedmetal layer is made of a high purity nickel containing nickel in acontent of 99.9 wt % or more, a nickel-phosphorus alloy or anickel-boron alloy.
 3. The wiring substrate of claim 2, wherein asurface of the lead-free electroless plated metal layer is coated with agold plated layer.
 4. A wiring substrate comprising: an insulator; awiring conductor made of a metal having a high melting point formed onthe insulator; an element of the platinum group precipitated on asurface of the wiring conductor; and a lead-free electroless platedmetal layer formed on the wiring conductor covering the element of theplatinum group existing on the surface of the wiring conductor, whereinthe element of the platinum group has been precipitated from a solutioncontaining the element of the platinum group and a hydroxy carboxylicacid, and has served as an initiating point to coat the surface of thewiring conductor with the lead-free electroless plated metal layer. 5.The wiring substrate of claim 4, wherein the insulator in made of analuminum oxide sintered body, an aluminum nitride sintered body, amullite sintered body, or silicon carbide sintered body, the wiringconductor is made of tungsten, molybdenum, or manganese, the lead-freeelectroless plated metal layer is made of a high purity nickelcontaining nickel in a content of 99.9 wt % or more, a nickel-phosphorusalloy, a nickel-boron alloy, copper, or an alloy containing copper as amain component.
 6. The wiring substrate of claim 4, wherein a surface ofthe wiring conductor with the lead-free electroless plated metal layeris coated with a gold plated layer.
 7. A wiring substrate comprising: aninsulator; a wiring conductor made of a metal having a high meltingpoint formed on the insulator; an element of Group 1B precipitated on asurface of the wiring conductor; and a lead-free electroless platedmetal layer formed on the wiring conductor covering the element of Group1B existing on the surface of the wiring conductor, wherein the elementof Group 1B has served as an initiating point to coat the surface of thewiring conductor with the lead-free electroless plated metal layer, theelement of Group 1B has been precipitated from a solution containing theelement of the Group 1B and a hydroxyl carboxylic acid.